1. Field of the Invention
The present invention is generally related to communication systems and in particular to wireless communication systems.
2. Description of the Related Art
Communication systems, and in particular, wireless communication systems comprise a plurality of communication links through which subscribers of such systems communicate with each other and with the system. A link typically comprises a plurality of communication channels such as signaling channels and traffic channels. Traffic channels are communication channels through which users of the communication system convey (i.e., transmit and/or receive) user information. Signaling channels are used by the system equipment to convey signaling information used to manage, operate and otherwise control the system. The system equipment, which are typically owned, maintained and operated by a service provider, are various well known radio and processing equipment used in communication systems. The system equipment along with user equipment (e.g., cell phone) generate and receive the signaling information.
Communication signals transmitted and received via communication links are often distorted by various anomalies that exist in the communication channels causing the signals to be received erroneously. The channel anomalies (e.g., fading, frequency translation, phase jitter) often cause the signals to lose power so that a signal is received at a significantly lower power level than it was transmitted. As a result, signals adversely affected by channel anomalies are often received with errors. One effective way of preventing errors from occurring or at least reduce the likelihood of errors occurring is the application of power control techniques by communication systems.
One power control techniquexe2x80x94called the outer loop power control schemexe2x80x94is often used by communication systems to adjust the transmission power level of transmitted signals to compensate for the effects of channel anomalies. For example, a signal being transmitted by a cell phone (or other user equipment), that is part of a Code Division Multiple Access (CDMA) wireless communication system, is received by a base station which measures the power level of the received signal and transmits it back to the cell phone. The base station sets a power threshold level and transmits this value to the cell phone to allow the cell phone to adjust (i.e., increase, decrease or maintain) the transmission power level of the signal such that it is received by the base station with a power level that is at or above the set threshold. The threshold is selected so that the base station receives the signal at an appropriate power level that tends to reduce the occurrence of errors. An outer loop power control scheme can thus be established in a reverse link; a reverse link is a communication link over which user equipment (e.g., cell phone, wireless laptop) transmits information to system equipment such as a base station. Also, a similar outer loop power control scheme can be established in a forward link; a forward link is a communication link over which system equipment transmits information to user equipment. Although the use of an outer loop power control scheme to address the problems caused by channel anomalies is reasonably effective, such a technique requires constant monitoring of the traffic channels. The power control scheme can be used in other types of wireless communication systems such as Time Division Multiple Access (TDMA) systems and Frequency Division Multiple Access (FDMA) systems.
Communication links in wireless communication systems such as CDMA systems comprise primary channels and secondary channels. Primary channels are communication channels in which communication signals are transmitted continuously; that is, even when no information is being conveyed over such primary channels, communication signals are being transmitted through such channels. Also, signaling information, which is used to establish, maintain and terminate communication between a user and system equipment, is conveyed only over primary channels. Once communication is established between a user and a base station, communication signals are conveyed continuously over the primary channels until the communication is terminated. Communication is established between a user and a base station when both the user and the base station follow procedures of a protocol for allocating system resources to the user to allow the user to use the communication system. The protocol is typically part of an established standard with which the communication system complies.
Secondary channels are additional channels that are mainly used to provide higher data rates to a user. For example, a user that is communicating voice over a primary channel may be provided with secondary channels to convey data. Typically, secondary channels convey only user information (e.g., data) and no signaling information. Secondary channels are often used in soft handoffs. In a soft handoff scenario, a user transmits information that is received by several base stations over a reverse link. The channel of the reverse link over which communication signals are being transmitted continuously and over which signaling information is conveyed is the primary channel. A reverse link (or forward link) may contain more than one primary channel. The other channels of the reverse link are the secondary channels. The primary channels and secondary channels are part of respective active sets of channels. The primary channels and the secondary channels consist of channel connections. The active set of primary channels and the active set of secondary channels are allocated to a particular user for established communications between the user and one or more base stations. The base stations (or a sector of a cell) are members of the active set; i.e., the members are mapped (i.e., connected) to channel connections. The number of members in the active sets of the primary and secondary channels are not necessarily equal. Communication channels that are assigned to the same members and the same user are said to be associated with each other; that is, communication channels that are in the same active set are associated with each other. Also, communication signals that are in different active sets but convey information from a common user and/or common members are also associated with each other. For example, a secondary channel can be associated with a primary channel; even though they are in different active sets, they convey information to the same user and the same members (i.e., base stations). Sometimes the secondary channel uses a subset of the primary channel active set, but typically the active sets share the same members.
The several base stations process the received user information and determine the quality of the received user information. The quality of the user information is often defined in terms of the Frame Error Rate (FER). User information is formatted as frames or blocks of information (e.g., blocks of bits). A frame that contains an error is called a bad frame or an erroneous frame. The ratio of the number of bad frames received to the total number of frames received for a defined period of time is called the Frame Error Rate. In soft handoff, several base stations process the received frames and the received frame that contains no errors (or the least amount of errors) is selected. Thus, the use of soft handoff tends to improve the FER of a user. However, the proper amount of power has to be allocated by the base stations and by the user equipment in order to transmit communication signals (containing frames of information) over both the primary and secondary channels. Also, transmission of communication signals over secondary channels introduces interference to other primary and secondary channels of other users.
In an effort to use power efficiently and reduce the occurrence of interference, communication systems have adopted a Discontinuous Transmission (DTX) protocol for the secondary channels. Unlike the primary channels, the secondary channels using DTX do not transmit signals continuously. In fact, there are periods when no signals are transmitted over such secondary channels. During such times, the interference due to the secondary channels in DTX mode is eliminated and no power is being expended unnecessarily. Typically, in DTX, the receiving equipment does not know when the transmitting equipment has entered DTX mode. Therefore, the receiving equipment has to rely on some type DTX mode detector. An example of a DTX mode detector is a device or method that measures the level of received power which is compared to a threshold. Typically, a DTX threshold is set by the receiving equipment and when signals being received over a secondary channel have a power level that is below the DTX threshold, the channel is declared to be in DTX mode. In DTX mode, received signals are not processed. However, DTX detectors are not fully reliable. In other words, a DTX detector may incorrectly label a signal as a DTX signal (meaning no signal transmitted) where in fact a signal was actually transmitted. Conversely, a DTX detector may fail to detect DTX (and label the received signal as a frame error) despite the fact that the transmitting equipment has entered DTX mode. Once DTX mode is declared, the outer loop power control is suspended and thus the transmission power level of transmitted signals is not adjusted. However, the use of DTX often leads to a problem called a deadlock state. A deadlock state occurs when received signals experience a deep fade (abrupt decrease in received signal power) that is incorrectly interpreted by receiving equipment as DTX mode. As a result, the quality of the secondary channel having signals experiencing deep fades steadily worsens (e.g., FER increases) because power for such signals is not adjusted and such signals are not even processed by the receiving equipment. The secondary channel can remain in the deadlock state indefinitely resulting in a great loss of information and link capacity. The quality of the secondary channel will continue to worsen and the channel will remain in this deadlock state until the power level of the signals rises above the DTX threshold at which point the outer power control scheme would again be applied. What is therefore needed is a method of practicing outer loop power control that prevents communication channels that can operate in DTX mode from entering a deadlock state.
The present invention provides a method that helps to prevent a communication channel that can operate in DTX mode from entering into a deadlock state. When it is determined that the communication channel is in DTX mode, an updated power threshold for the communication channel is calculated from (1) a power threshold previously established for the communication channel; (2) a power threshold previously established from an associated primary channel and (3) a current power threshold established for the associated primary channel. The established power thresholds are based on power threshold information for the communication channel and the associated primary channel. The updated power threshold for the communication channel is thus calculated by combining previously established power thresholds for the communication channel and an associated primary channel with a current power threshold for the associated primary channel. Once the updated power threshold is calculated, communication signals conveyed over the communication channel will have power levels at or above the updated power threshold. Therefore, the power level of communication signals conveyed over the communication channel will tend to be above a DTX threshold level for the communication channel thereby preventing the communication channel from entering into a deadlock state.